Electro-optic modulators are used for, by example, optical fiber communications, free-space interconnects, spatial light modulators, and optical computers. Typical electro-optic modulators require a large percentage of modulation of the optical fields with small driving voltages and currents. The largest electro-optic effect in semiconductor materials is the quadratic electroabsorption for interband transitions. In bulk semiconductors, at room temperature, the interband excitonic transition is broadened by phonon interactions, thereby reducing the maximum electro-optic effects. Room-temperature semiconductor modulators have thus typically relied on the enhancement of transition strengths and the reduction of broadening of quantum-confined excitons within multiple quantum well structures.
The following U.S. Patents are illustrative of various types of optical devices, including modulators: U.S. Pat. No. 2,692,950, "Valve for Infrared Energy"; U.S. Pat. No. 3,331,036, "Optical Wave Modulators and Attentuators"; U.S. Pat. No. 3,726,585, "Electrically Modulated Radiation Filters"; U.S. Pat. No. 4,115,747, "Optical Modulator Using a Controllable Diffraction Grating"; and U.S. Pat. No. 4,264,125, "Transmissive Surface Layer Effect Electro-Optic Device for use in Optical Modulators and the Like".
In U.S. Pat. No. 4,767,195, "System and Method for Encoding Information Onto an Optical Beam", D. M. Pepper describes the use of photorefractive material, such as GaAs and BaTiO.sub.3, in the construction of a phase conjugating mirror (PCM). In this regard reference is also made to a paper authored by D. M. Pepper entitled "Nonlinear optical phase conjugation", Optical Engineering, March/April 1982, Vol. 21, No. 2, pp. 156-183.
In U.S. Pat. No. 5,016,990, "Method of Modulating an Optical Beam", P. J. Dobson describes the use of an etalon having a quantum size effect confinement region for electrons and holes. The quantum size effect confinement region is comprised of a GaAs/AlGaAs superlattice or a multiple-quantum well structure. The use of quantum wires or quantum dots, in place of a quantum well structure, to form the quantum size effect confinement region is mentioned in column 8. The etalon is used to amplitude modulate an optical beam in accordance with a control optical beam.
As indicated in the foregoing U.S. Patent, efforts to improve electro-optic properties have been directed towards reducing the dimensionality of the excitons by bandgap engineering. Optically responsive media is one important application that benefits from a reduction in the exciton dimensionality. As an example, D. D. Nolte, D. H. Olson, G. E. Doran, W. H., Knox, and A. M. Glass describe, in an article entitled: Resonant photodiffractive effect in semi-insulating multiple quantum wells, J. Opt. Soc. Am. B, Vol. 7, No. 11, November 1990, pages 2217-2225, the use of semi-insulating multiple quantum wells to combine the holographic properties of the photorefractive effect with the large resonant optical nonlinearities of quantum-confined excitons. GaAs--AlGaAs multiple-quantum-well structures are made semi-insulating by proton implantation. The implant damage produces defects that are available to trap and store charge during transient holographic recording using coherent optical excitation. The application of these devices to image processing is demonstrated by the use of the Franz-Keldysh effect in four-wave mixing at wavelengths near 830 nm.
The following U.S. Patents are exemplary of various optical systems: U.S. Pat. No. 3,517,206, "Apparatus and Method for Optical Read-out of Internal Electrical Field"; U.S. Pat. No. 3,518,634, "Optical Memory with Photoactive Memory Element", U.S. Pat. No. 4,329,020, "Method of Manufacturing Inverse Filters by Holographic Techniques"; and U.S. Pat. No. 4,432,597, "Transmissive Holographic Optical Element on Aberatting Substrate".
In U.S. Pat. No. 3,660,818, "Electro-Optic Memory" J. J. Amodei and R. Williams describe the use of donor-doped large bandgap material, such as GaAs, having a compensator acceptor diffused into a surface. The compensator centers neutralize the effect of the donor impurities by trapping the free electrons contributed by the donor atoms. The effect is said to return the material to intrinsic values of resistivity.
All such optical storage media have one basic requirement: a permanent or semi-permanent optically induced change in the optical properties of the media. Many types of materials and processes satisfy this requirement.
As an example, the above-mentioned photorefractive materials can be employed to form optical storage media. In the photorefractive effect, photoinduced carriers drift or diffuse from a photoexcited region and are trapped at defects. The resulting trapped space-charge electric fields alter the refractive index of the materials through the electro-optic effect. The photorefractive effect relies on holographic techniques that produce interference fringes in the material.
A closely related phenomenon is the photodiffractive effect. In the photodiffractive effect, the trapped space-charge field pattern alters both the index of refraction and also the radiation absorption coefficient of the media, in addition to the refractive index. The photorefractive effect is the general effect of diffraction from a refractive index grating or an absorption grating.
One important consideration in realizing a high storage density holographic optical media is the spatial resolution of the media material, defined by a minimum interference fringe spacing. The storage time of the information into the media material is another important consideration.
Reference is made to U.S. Pat. No. 5,004,325, "Optical Processing Using a Multilayer Heterostructure", to A. M. Glass, W. H. Knox, and D. D. Nolte. This patent describes a multi-layered, multiple quantum well (MQW) electro-optic medium wherein deep levels are formed by proton implantation. The deep levels function to localize photocarriers. Deep levels are also said to be created by implanting oxygen atoms, neutron exposure, or by doping the heterostructure with certain impurities. For GaAs, chromium is said to be a suitable dopant for creating deep levels.
Heretofore, most electrooptic/photodiffractive storage materials are characterized as single phase materials having uncontrolled defect densities and properties. The single phase material absorbs light and generates photoexcited carriers such as electrons and holes. The charge carriers diffuse away from a region of high light intensity, or are driven away by an externally applied electric field, and are trapped at defects in the material. However, in that the defect states are difficult to control in density, and generally have a multiplicity of energy levels, the carriers trapped at such defects have uncontrolled lifetimes.
Low temperature grown (LTG) molecular beam epitaxy (MBE) GaAs has been suggested for use as a buffer layer for electrical isolation of integrated circuits, as a material in ultra-fast switches, and as a material for constructing long-wavelength radiation detectors. As an example of the first application, reference is made to an article by F. W. Smith, A. R. Calawa, Chang-Lee Chen, M. J. Manfra, and L. J. Mahoney, "New MBE Buffer Used to Eliminate Backgating in GaAs MESFET's", IEEE Electron Device Letters, Vol. 9, No. 2, 2/88, pp. 77-80, and an article by M. R. Melloch, D. C. Miller, and B. Das, "Effect of a GaAs buffer layer grown at low temperatures on a high-electron-mobility modulation-doped two-dimensional electron gas" Appl. Phys Lett. 54(10), 6 Mar. 1989, pp. 943-945.
Commonly assigned U.S. patent application Ser. No. 07/715,757, filed Jun. 14, 1991, "Compound Semiconductor Having Metallic Inclusions and Devices Fabricated Therefrom" by J. Burroughes, R. R. Hodgson, D. T. McInturff, M. R. Melloch, N. Otsuka, P. M. Solomon, A. C Warren, and J. N. Woodall (abandoned in favor of Ser. No. 08/104,423, filed Aug. 9, 1993 (allowed)) describes the use of LTG GaAs:As in constructing radiation detectors, such as PIN diodes. It is shown that the As precipitates can absorb light of less than bandgap energy, and that the GaAs:As material can be employed as a photodetector for sub-bandgap light. The LTG GaAs material is grown by MBE techniques at relatively low substrate temperatures (200.degree. C.), resulting in approximately one percent of excess arsenic being incorporated into the GaAs material. During post-growth annealing, the excess arsenic has been found to precipitate into clusters, forming GaAs:As.
The following articles are referenced for describing the fabrication and physical properties of LTG GaAs:
Optically Induced Reordering of As Cluster Defects In Semi-insulating GaAs, by J. Jimenez, P. Hernandez, M. A. Gonzalez, L. F. Sanz, and J. A. De Saja, Cryst. Latt. Def. and Amorph. Mat., Vol. 17, 1987 pp. 199-204; PA1 Structural Properties of As-rich GaAs Grown by Molecular Beam Epitaxy at Low Temperatures, by M. Kaminska, Z. Liliental-Weber, E. R. Weber, T. George, J. B. Kortright, F. W. Smith, B-Y. Tsaur, and A. R. Calawa, Appl. Phys. Lett. 54 (19), pp. 1881-1883, 8 May 1989; PA1 Arsenic Precipitates and the Semi-insulating Properties of GaAs Buffer Layers Grown by Low-temperature Molecular Beam Epitaxy, by A. C. Warren, J. M. Woodall, J. L. Freeouf, D. Grischkowsky, D. T. McInturff, M. R. Melloch, and N. Otsuka, Appl. Phys. Lett. 57(13), pp. 1331-1333, 24 Sep. 1990; and PA1 Formation of Arsenic Precipitates in GaAs Buffer Layers Grown by Molecular Beam Epitaxy at Low Substrate Temperatures, by M. R. Melloch, N. Otsuka, J. M. Woodall, A. C. Warren, and J. L. Freeouf, Appl. Phys. Lett. 57(15), pp. 1531-1533, 8 Oct. 1990.
Reference is also made to an article entitled "Low-temperature-grown GaAs quantum wells: Femtosecond nonlinear optical and parallel-field transport studies", by W. H. Knox, G. E. Doran, M. Asom, G. Livescu, R. Leibenguth, and S.N.G. Chu, Appl. Phys. Lett. 59(12), pp. 1491-1493, 16 Sep. 1991. These authors report a broadening of the exciton absorption by the presence of excess arsenic defects within a LTG structure having 75 periods of 6.0 nm GaAs wells and 3.0 nm barriers. The sample was mounted on a glass substrate and the back etched away by a selective chemical etching.
What is not disclosed by the foregoing U.S. Patents and journal articles, and what is thus an object of this invention to provide, is a non-linear heterogeneous optical material that is particularly useful as a high density optical storage medium; the heterogeneous material being comprised of a Group III-V material having Group V precipitates contained therein. The material may also be comprised of, by example, Ge having Ni precipitates.
A further, related object of this invention is to provide a non-linear heterogeneous optical material that is particularly useful as a high density optical storage medium; the heterogeneous material being comprised of a Group III-V material having Group V precipitates contained therein; wherein the precipitates confine the wavefunctions of electrons and holes, resulting in an "exciton in a cage" effect for excitons within the Group III-V material between the Group V precipitates.
Another object of this invention is to provide an optically responsive heterogeneous material having a plurality of phases, wherein at least one phase absorbs light and generates photoexcited carriers, at least one phase captures the photoexcited carriers, and at least one phase is a dielectric phase having optical constants which are a function of a local electric field.
Another object of this invention is to provide an optical storage medium that overcomes the problems of the prior art, that exhibits a high optical storage density, and that operates with low optical power.
Another object of this invention is to provide an optical storage medium that is comprised of a Group III-V material and that does not require the growth of multiple quantum wells, thereby providing lower cost and reduced fabrication complexity.
A further object of this invention is to provide an optical storage medium that is comprised of a Group III-V heterogeneous material having Group V precipitates contained therein, the precipitates function to confine excitons within a "cage" of precipitates.
A further object of this invention provides an information storage and retrieval system that includes an optical storage medium that is comprised of a Group III-V heterogeneous material having Group V precipitates contained therein.
Another object of this invention is to provide an optically responsive material that exhibits photorefractive gain.